(a) Technical Field
The present invention relates to a wound rotor synchronous motor, more particularly, to a rotor structure of a wound rotor synchronous motor capable of reducing torque ripple while ensuring mechanical strength due to centrifugal force of a rotor coil.
(b) Description of the Related Art
In general, a hybrid vehicle or an electric vehicle may generate a driving torque by an electric motor (hereinafter referred to as “drive motor”) which obtains a driving torque from electrical energy.
For example, a hybrid vehicle can be driven in an electric vehicle (EV) mode, which is a pure electric vehicle mode using only power of a drive motor, or in a hybrid electric vehicle (HEV) mode using both torque of an engine and torque of the drive motor as power. Generally, an electric vehicle is driven using torque of the drive motor as the power source.
In particular, the drive motor, which is used as a power source of an environmentally friendly vehicle, generally uses a permanent magnet synchronous motor (PMSM). The PMSM maximizes the performance of a permanent magnet in a limited layout condition.
In the above permanent magnet, neodymium (Nd) improves intensity of a permanent magnet, and dysprosium (Dy) improves high temperature demagnetization resistance. However, rare earth metals Nd and Dy embedded in the permanent magnet are limited to some of countries, such as China, and Nd and Dy are very expensive and have large price fluctuations.
In order to address the above-mentioned problems, an induction motor for a hybrid vehicle has been developed. However, in order to represent the same motor performance, the induction motor undesirably has an increased volume and weight.
Meanwhile, in recent years, as a drive motor serving as a power source of an environmentally friendly vehicle, a wound rotor synchronous motor (hereinafter referred to as “WRSM”) has been developed to replace a permanent magnet synchronous motor (PMSM).
In a state that a coil is wound around a rotor as well as a stator, if a current is applied to the WRSM, the rotor is electromagnetized so that a permanent magnet of a permanent magnet synchronous motor (PMSM) may be replaced with the WRSM.
In the WRSM, a rotor is spaced apart from an inner side of a stator by a predetermined gap. If power is applied to a coil of the stator and a coil of the rotor, a magnetic field is formed. The rotor is rotated by magnetic attraction generated between the coil of the stator and the coil of the rotor.
Unlike the permanent magnet synchronous motor, since the coil is wound around the rotor in a WRSM, great centrifugal force is applied to the coil of the rotor upon high speed rotation (generally a maximum of 10,000 rpm or greater in a case of an EV).
Accordingly, in the WRSM, upon high speed rotation of the rotor, ordering of the coil of the rotor may be deteriorated due to the centrifugal force applied to the coil of the rotor, and stress is concentrated to a rotor tooth winding the coil of the rotor so that the rotor tooth may be broken.
Meanwhile, torque is a parameter to determine the performance of the WRSM. Since torque ripple is closely related to vibration noise, the torque ripple may be very important. The torque and the torque ripple may be determined according to shapes of a stator and a rotor, and a winding fill factor and a current of a coil.
In the meantime, the performance may be determined according to a shape of teeth winding the rotor coil. If an end of the teeth is short, a magnetic flux is concentrated thus ensuring the torque is advantageous, but torque ripple is increased. Upon high speed rotation of the rotor, it is difficult to ensure mechanical strength for supporting centrifugal force of the stator coil.
Accordingly, in the conventional art, a wedge is inserted between teeth so that mechanical strength for supporting a centrifugal force of the rotor coil is ensured, but the number of components is increased.
Further, in the conventional art, there is a method of distributing the centrifugal force applied to the rotor coil by increasing a contact area of the rotor coil by increasing lengths of ends of teeth. However, such an approach deteriorates the torque performance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.